Omnidirectional photoacoustic tomography system

ABSTRACT

An ultrasonic photoacoustic tomography system includes a mirror arrangement configured to redirect an incoming light beam defining a central axis such that the mirror arrangement reflects the incoming light beam to form a converging ring-shaped light beam. This converging ring-shaped light beam directs light originating from the incoming light beam radially inward covering a 360° circumferential range around the central axis. A closed-geometry acoustic detector is configured to pick up reactive sound waves from tissue irradiated by the converging ring-shaped light beam over the 360° circumferential range.

TECHNICAL FIELD

The present application relates to a system for photoacoustictomography.

BACKGROUND

Photoacoustic tomography (optoacoustic imaging) is a biomedical imagingmodality technology utilizing the photoacoustic effect. In photoacoustictomography, non-ionizing laser pulses are delivered into biologicaltissues. Some of the delivered energy will be absorbed and convertedinto heat, leading to transient thermoelastic expansion and thuswideband (i.e. MHz) ultrasonic emission. The generated ultrasonic wavesare detected by ultrasonic transducers and then analyzed to produceimages. It is known that optical absorption is closely associated withphysiological properties, such as hemoglobin concentration and oxygensaturation. As a result, the magnitude of the ultrasonic emission (i.e.photoacoustic signal), which is proportional to the local energydeposition, reveals physiologically specific optical absorptioncontrast. 2D or 3D images of the targeted areas can then be formed.

Major existing methodologies in the field of ultrasonic photoacousticimaging, for example for scanning of breast tissue, are known asphotoacoustic mammoscope, LOUISA breast imaging, and as a single-lightsource excitation/cup-shape acquisition photoacoustic tomography systemby Optosonics Inc.

Existing photoacoustic systems not yet provide resolution andsensitivity needed for breast cancer screening. Existing technologiesrely on localized illumination of breast tissue.

SUMMARY

It is an object of the present application to describe an ultrasonicphotoacoustic tomography system that provides a high resolution andsensitivity without increasing the time duration of in vivo tissueexamination.

This objective is achieved by an ultrasonic photoacoustic tomographysystem comprising a mirror arrangement configured to redirect anincoming light beam defining a central axis such that the mirrorarrangement reflects the incoming light beam to form a convergingring-shaped light beam. This converging ring-shaped light beam directslight originating from the incoming light beam radially inward coveringa 360° circumferential range around the central axis. A closed-geometryacoustic detector is configured to pick up reactive sound waves fromtissue irradiated by the converging ring-shaped light beam over the 360°circumferential range. A closed geometry in this context may be a ringor any other geometry surrounding a central opening.

Preferably, both the converging ring-shaped light beam and the acousticdetector are configured to be movable along the central axis forscanning an object along an axial path.

In a preferred embodiment, the mirror arrangement includes at least afirst mirror with a first mirror surface being cone-shaped, a secondmirror being ring-shaped with a second mirror surface forming a hollowtruncated cone, thus forming a light bear in the shape of a hollowcylinder, and a third mirror being ring-shaped with a third mirrorsurface forming a hollow truncated cone, wherein the first, second, andthird mirrors are arranged coaxially with the central axis.

A mounting platform that is transparent for a wavelength bandwidth ofthe incoming light beam may be provided for holding the first mirror sothat the propagating light is uninterrupted by any mounting structure.

The mounting platform preferably extends in a radial plane and may alsohold the second mirror. In this arrangement, the mounting platform ispreferably positioned between the second mirror and the third mirror.

While the mounting platform, the first mirror, and the second mirror maybe in a fixed relationship relative to one another, the third mirror ispreferably axially movable relative to the mounting platform, the firstmirror, and the second mirror.

The closed-geometry acoustic detector may also be axially movablerelative to the mounting platform, the first mirror, and the secondmirror, together with or independently from the third mirror.

A drive mechanism may be provided for driving the acoustic detector andthe third mirror along the central axis.

An adjustment mechanism for axially guiding the third mirror may be heldby the mounting platform.

If the acoustic detector and the third mirror have a central openingwith a diameter of at least 150 mm, the arrangement is suitable, forexample, for breast tissue scanning.

In a further preferred embodiment, the first mirror surface has an apexconfigured to face the incoming light beam and to reflect the incominglight beam radially outward over the 360° circumferential range, thesecond mirror surface has an angle relative to the central axisconfigured to reflect the radially outward reflected light beam axiallyaway from the incoming light beam, and the third mirror surface has anangle relative to the central axis configured to reflect the axiallyreflected light beam radially inward to form the converging ring-shapedlight beam. For best detection of the photoacoustic effect, the radiallyinward reflected light beam encloses an angle of 30° to 80° with theincoming beam such that the illuminated area on a surface of an objectto be scanned is in an axial location overlapping with an axial positionof the closed-geometry acoustic detector.

The first mirror surface, the second mirror surface, and the thirdmirror surface are preferably first-surface mirror surfaces to preventrefractive aberrations.

An ultrasound source and ultrasound detector can be provided todetermine a distance of a surface of an object to be scanned from theconverging ring-shaped light beam.

With such a determination, the photoacoustic system can be configuredfor performing an automatic scanning procedure by adjusting a lightintensity of the incoming beam based on the determined distance.

The ultrasound detector can be combined with the acoustic detector.

Further benefits of the proposed photoacoustic tomography system willbecome evident from the following description of preferred embodimentsshown in the drawings.

The drawings are provided herewith solely for illustrative purposes andare not intended to limit the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 shows a schematic illustration of a first mirror arrangement foran omnidirectional photoacoustic tomography arrangement;

FIG. 2 shows a schematic illustration of a second mirror arrangement foran omnidirectional photoacoustic tomography arrangement;

FIG. 3 shows a schematic set-up of an omnidirectional photoacoustictomography arrangement using the second mirror arrangement; and

FIG. 4 is a graph of a test scan of simulated tissue performed with aset-up similar to that of FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show two schematic examples of the proposedomnidirectional photoacoustic tomography system 10 and 110,respectively. The term “omnidirectional” refers to a simultaneousring-shaped illumination as opposed to a parallel light beam circulatedaround an object to be scanned.

FIG. 1 shows a simplified schematic model of a proposed omnidirectionalphotoacoustic tomography system 10. It comprises a first mirror 12 witha cone-shaped outer mirror surface 14 that has an apex 16 facing anincoming collimated light beam 18. The incoming light beam 18 isreflected radially outward by the cone-shaped mirror surface 14 over acomplete 360° circumferential range around the central axis A defined bythe collimated light beam 18. Upon the reflection by the first mirror12, the light beam forms a generally planar light sheet 20 extendingradially outward from the first mirror 12. The light intensity of thelight sheet 20 decreases proportionally with the distance from thecentral axis A. A second mirror 22 is arranged in the path of the lightsheet 20. The second mirror 22 is ring-shaped with an inner mirrorsurface 24 shaped like a partial hollow cone. The mirror surface 24 ofthe second mirror 122 has a slope relative to the central axis A that isless than 45° to create an incidence angle and a reflection angle thatare both smaller than 45° to reflect the light sheet 20 radially inwardfor a ring-shaped illumination of any object to be scanned that isplaced in a location along the central axis A. However, an axialadjustment of the ring-shaped illumination of the object would involve amovement of the object to be scanned, and an additional repositioningmechanism for the object or for the complete mirror system would berequired.

FIG. 2 shows a second, functionally advanced, example of a proposedomnidirectional photoacoustic tomography system 110 that is alsoillustrated in more detail in FIG. 3. Like the example of FIG. 1, itincludes a cone-shaped mirror surface 114 of a first mirror 112 that hasan apex 116 facing an incoming collimated light beam 118. The incominglight beam 118 is reflected radially outward by the cone-shaped mirrorsurface 14 over a complete 360° circumferential range around the centralaxis A defined by the collimated light beam 118. Upon the reflection bythe first mirror 112, the light beam 118 forms a generally planar lightsheet 120 extending radially outward from the first mirror 112. Aring-shaped second mirror 122 surrounding the first mirror 112 ispositioned in the path of the light sheet 120.

Both the first mirror 112 and the second mirror 122 are mounted on atransparent mounting platform 126. The material of the mounting platform126 is chosen to be transparent to the wavelength band of the incomingcollimated light beam 118 that is used to perform the photoacoustictomography. In this context, “transparent” means that the material ofthe mounting platform 126 allows for a transmission of at least 50%, butpreferably 90%, of the light intensity within a given wavelength band.The configuration of the mounting platform 126 (shown in FIG. 3) allowsfor holding the first mirror 112 in place without obstructing the lightreflected by the second mirror 122. Should the incoming light beam 118include wavelengths that are not used, the mounting platform 126 may bean optical filter that is not transparent for all wavelengths of theincoming light beam 18, but that instead may filter out (by reflectionor absorption) such wavelengths that do not contribute to the excitationof a usable acoustic signal.

Because any refractive redirection of light causes chromaticaberrations, it is preferred that the incidence angle a and thereflection angle β of the second mirror 122 are identical to those ofthe first mirror 112, i.e. that the mirror surface 124 of the secondmirror 122 is parallel to the mirror surface 114 of the first mirror112. Thus, in the event that the light sheet 120 originating from thefirst mirror 112 is not entirely planar (but cone-shaped), the secondmirror 122 preferably redirects the light to a direction parallel to thecentral axis A to form a light cylinder 128 of constant diameter. Thisensures that the light cylinder 128 progresses perpendicular to thesurface of the mounting platform 126 so that both the incidence angleand the refractive angle of the light passing through the mountingplatform 126 are 0°.

In addition to the first mirror 112 and the second mirror 122, thephotoacoustic tomography system 110 of FIG. 2 includes a third mirror130. The third mirror is ring-shaped with a mirror surface 132 alsoforming a hollow truncated cone, which in this case encloses an acuteangle with the central axis A and radially reflects the light cylinder128 radially inward toward an object 134 to be scanned. As indicated inFIG. 2, the angle between the central axis A and the slope of the mirrorsurface of the third mirror 130 is less than 45°, for example between15° and 40° so that the radially inward reflected light encloses a totalangle ε of 30° to 80° with the incoming light beam 18, corresponding toan incidence angle γ and a reflection angle δ of 50° to 75° each. Thismirror slope thus results in both the incidence angle y and thereflection angle δ being greater than 45°. In effect, the light cylinder128 is reflected to form a cone shape with the apex of the light cone136 being axially farther removed from the first and second mirrors 112and 122 than the third mirror 130. This light cone 136 is projected onthe object 134 to be scanned to form an illuminated ring or sleeveextending over the entire 360° angular range around the central axis A.

The second and third mirrors 122 and 130 preferably have conical shapes.However, parabolic, spherical, or other profiles can also be used forcontrolling the light intensity distribution on the object 134.

The geometry of the third mirror 130 as shown has certain advantages aswill become evident from the following description of FIG. 3.

In the example of FIG. 3, the angle of the first mirror surface 114 andof the second mirror surface 124 are shown at 45° to provide incidenceangles and reflection angles of 45°. But as explained above, otherangles are feasible as well, preferably in an arrangement, in which alight cylinder 128, which propagates parallel to the central axis A,passes through the radial mounting platform 126 at a right angle.

The third mirror 130 is axially movable via an adjustment mechanism 138that does not require any movement of the first mirror 12 or the secondmirror 122, nor a movement of the object 134 to be scanned. The shownadjustment mechanism 138 has a guiding structure 142 for axially guidingthe third mirror 130 along the central axis A. Arrows indicate a drivemechanism 144 for moving the third mirror 130 via the adjustmentmechanism 138.

Notably, as common in optical arrangements, all mirrors are preferablyfirst-surface mirrors. Accordingly, all optical elements redirecting theincoming light beam 118 to form the converging ring-shaped light beamemitting the light cone 136, i.e. the first mirror 12, the second mirror122, and the third mirror 130, are reflective and not refractive. As aresult, all undesirable optical aberrations are eliminated.

Axially adjacent the third mirror 130, a closed-geometry ultrasoundtransducer 140 is positioned. In the shown embodiment, the ultrasoundtransducer 140 is ring-shaped and has its own adjustment mechanism 146and drive mechanism 148 that may structurally resemble the drivemechanism 144 and adjustment mechanism 138 of the third mirror 130. Aspreviously mentioned, the ultrasound transducer 140 may have a differentclosed geometry and may, for example, form a hollow square or anotherpolygonal frame.

For the function of the photoacoustic tomography system 110, it may befeasible to provide only one drive mechanism 144 and one adjustmentmechanism 138 for jointly moving the third mirror 130 and the ultrasoundtransducer 140. Where a joint drive mechanism 144 is used, theultrasound transducer 140 may require a greater axial length ofsensitivity because the illuminated ring of small-diameter objects 134would be farther removed from the third mirror 130 than the illuminatedring of an object 134 with a large diameter. A movement of theultrasound transducer 140 independent from the movement of the thirdmirror 130 as shown allows the axial position of the ultrasoundtransducer 140 to be adjusted relative to the third mirror 130, based onthe diameter of the scanned object 134.

The photoacoustic tomography system 110 may include a water-filledcontainer 150 with a transparent bottom formed by the mounting platform126 and with a surrounding wall 152 sealingly secured to the mountingplatform 126. The container 150 is preferably filled with water up to awater level 154 above the third mirror 130 and the enclosed ultrasoundtransducer 140 because air-to-glass and glass-to-water refractions willnot affect the cylindrical beam formation. The guiding structure 142 isshown to be sealingly extending through the container wall 152 to theoutside of the container, where it may be anchored to a supportingstructure that is not shown. Alternatively, the guiding structure 142may be mounted on the inside of the container, for example to themounting platform 126 or to the container wall 152 so that no containeropenings are required.

The closed-geometry ultrasound transducer 140 preferably has a dualfunction. On the one hand, it detects the acoustic signals caused in anobject 134 by the light cone 136. On the other hand, the ultrasoundtransducer 140 performs the sonic determination of the distanced betweenthe object 134 and the third mirror 130. For the sonic determination,ultrasound waves 156 are emitted radially inward from theclosed-geometry ultrasound transducer 140. The acoustic detector in theultrasound transducer 140 receives the reflected ultrasound waves anddetermines the distance d of the object 134 in a manner known per se.

The distance information is then fed to a scanning algorithm that notonly determines the axial length, along which the scanning process isperformed, but also the light intensity required for uniformillumination at different axial positions, and further the requiredrelative axial positions of the ultrasound transducer 140 and the thirdmirror 130 with respect to each other. The intensity adjustment takesinto account that, in a converging light wave such as the light cone136, the intensity increases closer to the center of convergence and canpartly compensate for light attenuation due to absorption or scatteringlosses.

For example, the sonic determination may be performed in a firstscanning process, followed by the photoacoustic tomography scanningprocess that is calibrated based on the parameters acquired during thefirst scanning process. Alternatively, a single axial movement mayalternate between the sonic determination of the distance d and thephotoacoustic tomography scanning process in quick succession. Ofcourse, the ultrasound transducer 140 may be a separate element from theacoustic detector for the photoacoustic tomography system 110 withoutleaving the scope of the present invention.

It is further contemplated that the photoacoustic tomography system 110of FIG. 3 is arranged in the orientation as shown, which means that thecentral opening 158 of the ultrasound transducer 140 forms an upperinsertion opening for the object 134 to the scanned and that theincoming light beam 118 originates from below the shown structure. Thishas the benefit that gravity will promote a proper arrangement of theobject 134 along the central axis A without requiring additional supportstructures inside the photoacoustic tomography system 110. If thephotoacoustic tomography system 110 is to be used for breast scanning,the central opening 148 of the ultrasound transducer 140 and of thethird mirror 130 is preferably dimensioned to be large enough to inserta human breast for breast cancer scanning. Thus, for a breast scanner,the opening should have a diameter D of at least 150 mm.

The light source used for the photoacoustic tomography scanning processis chosen based on the material of the object 134 to be scanned. Thewavelengths for photoacoustic tomography are typically within thevisible and near infrared spectrum for body tissue, but may be within adifferent spectral range for different materials of the object 134 to bescanned. Thus, the proposed photoacoustic tomography system 110 may be amodular addition to an existing photoacoustic tomography system thatincludes a feasible light source for a given purpose, or thephotoacoustic tomography system 110 may be built as a stand-alone unitincluding one or more lasers suited for the intended application orapplications.

The proposed photoacoustic tomography system 110 ensures a fullyenclosed light path that does not require any shielding fromenvironmental noise, be it electromagnetic or acoustic waves. As thelasers used for photoacoustic tomography scanning are pulsed, anybackground noise can be measured between pulses and be subtracted fromthe yielded acoustic signal. This allows for easy access to the spacebetween the second mirror 122 and the third mirror 130 for properpositioning of the object 134 prior to the scanning process.

Accordingly, the present application provides a novel illuminationsystem for enhanced photoacoustic tomography with all-reflective,aberration-independent, adjustable optics for generating a ring-shapedillumination pattern on an object to be scanned.

The described full-ring illumination and detection provides a platformfor fast, optimized photoacoustic tomography with enhanced penetrationdepth without the need for painful tissue compression. By simultaneouslyscanning entire rings or slices of tissue, the scanning process isgreatly accelerated compared to existing photoacoustic tomographysystems. The closed geometry of the acoustic transducer elements allowsfor a more efficient acquisition of the omnidirectional photoacousticsignals.

Prototype

A small-scale prototype to confirm the feasibility of omnidirectionalillumination has been assembled and tested using off-the-shelf parabolicreflectors in lieu of conical ring mirrors. To initially test the effectof ring illumination, preliminary photoacoustic studies were conducted.Due to the small-size of the prototype optics (50 mm ring diameter) andlimitations of integrating the small size prototype with actual ringacoustic detection, the experiments were designed as follows: aconverging cylindrical shape illumination was generated by adjusting thedistance between the first mirror and the second mirror. A cone-shapetissue-mimicking phantom, made out of 10% porcine gelatin (300 bloom)and 50 pm cellulose particles, was added to create “light scattering” aswell as an “acoustic backscattered” medium. An optical absorber (700 μmpencil lead) was embedded within the phantom and oriented diagonally sothat light absorbing points would be distributed at different distancesfrom the outer surface of the phantom. The converging ring illumination,combined with a geometry of the phantom, simulated the generation of aring on a cylindrical breast shape. A programmable US scanner(Verasonics Vantage 128), equipped with a linear array transducer(L11-4v) was utilized to acquire US and photoacoustic signals along thediameter of the phantom.

The sliced illumination generated by ring illumination and mediumdiffusion is shown in the graph of FIG. 4. In the graph, thephotoacoustic signal intensity is plotted (in arbitrary units with themaximum signal set to “1”) across the diameter of the phantom (i.e.distance from the outer surface of the phantom in mm). The laser energywas set to be 10 mJ/pulse (at 532 nm).

As anticipated, the photoacoustic signal intensity is elevated justbelow the surface layer due to the high density of photons. However, thenormalized photoacoustic amplitude clearly shows that passing thehigh-fluence superficial layer, the photoacoustic signal remained quiteuniform over a range of distances between 10 and 22 mm, indicating thatomnidirectional ring illumination generated a relatively uniform lightenergy deposition with respect to depth. The photoacoustic signalvanished after 25 mm because the diagonal placement of the pencil leadplaced it outside of the illuminated region. This also demonstrated thatwith ring illumination, it is possible to focus the laser energy withina specific slice of the tissue instead of illuminating the whole object.

Advantages

The proposed UST/PAT system is different and advantageous compared toexisting technologies. For example, compared to a PA mammoscope asdeveloped by the University of Twente in cooperation with Canon Inc.,the presented system does not require breast compression. The main issuewith mammographic photoacoustic configurations, in addition to thediscomfort, is that breast compression causes the blood to rush out,which adversely affects photoacoustic performance. Blood is the majorendogenous photoacoustic contrast medium and biomarker. Compared toLOUISA 3D breast imaging developed by Tomowave or a photoacoustictomography system by Optosonics, which utilizes a single-light-sourceexcitation and a cup-shaped acquisition, the presented method isexpected to have an improved penetration depth and a more uniformillumination pattern. This assumption is based on preliminary in-silicoMonte Carlo simulation and on the above described tissue mimickingphantom study using a prototype.

Table 1 below provides a comparative overview:

TABLE 1 Comparison between proposed UST/PAT and competing technologiesImaging hemo- Imaging globin modalities/ SOS, AA, and Breast relatedimage Fluence Coverage bio- repre- compensation (areas close markerssentation ability to chest wall) Proposed Yes 3 D US, PA, Yes Yes systemElasticity PA Not 2 D PA No Yes Mammoscope efficient LOUISA 3 D Yes 2 DUS, No Yes system 3 D PA Optisonics Yes 3 D PA No No System

In addition, accurate ultrasound tomography imaging via the ultrasoundtransducer, including speed-of-sound maps and acoustic-attenuation mapsmay be used prior to the photoacoustic tomography performed by thesystem described above for a precise measurement of the acousticheterogeneity of the breast tissue. The preceding ultrasonicmeasurements provide information for implementing advancedreconstruction algorithms to compensate for acoustic heterogeneity ofthe breast tissue that may be indicative of non-uniform light fluence.Such advanced and model-based reconstruction methods significantlyenhance the quality of photoacoustic tomography in terms of sensitivityand resolution and, more importantly, enables quantitative photoacousticimaging.

While the above description constitutes the preferred embodiments of thepresent invention, it will be appreciated that the invention issusceptible to modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

1. A photoacoustic tomography system comprising a mirror arrangement configured to redirect an incoming light beam defining a central axis, the mirror arrangement reflecting the incoming light beam to form a converging ring-shaped light beam, which directs light originating from the incoming light beam radially inward covering a 360° circumferential range around the central axis, and a closed-geometry acoustic detector configured to pick up reactive sound waves an object irradiated by the converging ring-shaped light beam over the 360° circumferential range.
 2. The photoacoustic tomography system of claim 1, wherein both the converging ring-shaped light beam and the acoustic detector are configured to be movable along the central axis.
 3. The photoacoustic tomography system of claim 1, wherein the mirror arrangement includes at least a first mirror with a first mirror surface being cone-shaped, a second mirror being ring-shaped with a second mirror surface forming a hollow truncated cone, and a third mirror being ring-shaped with a third mirror surface forming a hollow truncated cone, wherein the first, second, and third mirrors are arranged coaxially with the central axis.
 4. The photoacoustic tomography system of claim 3, further comprising a mounting platform that is transparent for a wavelength bandwidth of the incoming light beam, the mounting platform holding the first mirror.
 5. The photoacoustic tomography system of claim 4, wherein the mounting platform extends in a radial plane and also holds the second mirror.
 6. The photoacoustic tomography system of claim 5, wherein the mounting platform is positioned between the second mirror and the third mirror.
 7. The photoacoustic tomography system of claim 4, wherein the mounting platform, the first mirror, and the second mirror are in a fixed relationship relative to one another and the third mirror is axially movable relative to the mounting platform, the first mirror, and the second mirror.
 8. The photoacoustic tomography system of claim 7, wherein the closed-geometry acoustic detector is axially movable relative to the mounting platform, the first mirror, and the second mirror.
 9. The photoacoustic tomography system of claim 8, wherein the closed-geometry acoustic detector is coupled to the third mirror and movable therewith.
 10. The photoacoustic tomography system of claim 9, further comprising a drive mechanism configured for driving the acoustic detector and the third mirror along the central axis.
 11. The photoacoustic tomography system of claim 7, further including an adjustment mechanism for axially guiding the third mirror, the adjustment mechanism being held by the mounting platform.
 12. The photoacoustic tomography system of claim 3, wherein the acoustic detector and the third mirror have a central opening with a diameter of at least 150 mm.
 13. The photoacoustic tomography system of claim 3, wherein the first mirror surface has an apex configured to face the incoming light beam and to reflect the incoming light beam radially outward over the 360° circumferential range, the second mirror surface having an angle relative to the central axis configured to reflect the radially outward reflected light beam axially away from the incoming light beam, and the third mirror surface having an angle relative to the central axis configured to reflect the axially reflected light beam radially inward to form the converging ring-shaped light beam, wherein the radially inward reflected light beam encloses an angle of 30° to 80° with the incoming beam.
 14. The photoacoustic tomography system of claim 13, wherein the third mirror surface is arranged at an angle relative to the central axis configured to redirect the axially reflected light beam toward an axial location on a surface of the irradiated object, the axial location overlapping with an axial position of the closed-geometry acoustic detector.
 15. The photoacoustic tomography system of claim 3, wherein the first mirror surface, the second mirror surface, and the third mirror surface are first-surface mirror surfaces.
 16. The photoacoustic tomography system of claim 1, further comprising an ultrasound source and ultrasound detector configured to determine a distance of a surface of the irradiated object from the central axis.
 17. The photoacoustic tomography system of claim 16, wherein the photoacoustic tomography system is configured for performing an automatic scanning procedure by a adjusting a light intensity of the incoming beam based on the determined distance.
 18. The photoacoustic tomography system of claim 16, wherein the ultrasound detector is formed by the closed-geometry acoustic detector.
 19. The photoacoustic tomography system of claim 1, further comprising a light source producing a dimmable collated light beam.
 20. The photoacoustic tomography system of claim 1, further comprising: an ultrasound source and ultrasound detector configured to determine an acoustic heterogeneity of the irradiated object; and a processor configured to process reconstruction algorithms to compensate amplitudes of the detected reactive sound waves for the acoustic heterogeneity of the irradiated object. 